Whenever the Secretary shall find, from the recommendations, information and evidence submitted by the committee, or from other available information, that to limit the shipment of potatoes to particular grades, sizes, and qualities thereof in any or all portions of the production area would tend to effectuate the declared policy of the act, he shall so limit by appropriate regulations thereon the shipments of such potatoes during a specified period. The Secretary shall notify the committee of any such regulation and the committee shall give reasonable notice thereof to handlers: Provided, That no regulations shall be issued under this subpart limiting the shipment of U.S. No. 1 grade or better, as such grades are defined in United States Standards for Potatoes in effect at the time such regulations are issued.